Method of forming objects from thermosensitive composition

ABSTRACT

Three dimensional objects are formed by scanning a liquid thermosensitive resin with a laser beam causing imagewise heating of the resin. Because thermosensitive compositions do not obey the law of linear superposition, the problem of stray exposure is eliminated allowing the thermal polymerization of any point within the volume of the liquid without affecting adjacent points.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part to an application entitled“Method of Forming Objects from Thermosensitive Composition”U.S. patentapplication Ser. No. 09/317,076 filed on May 18, 1999, now U.S. Pat. No.6,214,276.

FIELD OF THE INVENTION

The invention relates to the generation of three-dimensional objects byimaging, a field also known as stereolithography, and in particular tothermal stereolithography of various objects, including flexographicprinting plates.

BACKGROUND OF THE INVENTION

The generation of three dimensional (3D) objects by imaging a liquidresin is well known and has been commercially available for many years.Typically the liquid resin is made to polymerize in areas exposed tointense UV light from a laser or a mask illuminated by a UV lamp. Thetwo best known applications are building 3D models by a process known asstereolithography and manufacturing flexographic printing plates.Flexographic printing plates are printing plates having considerablesurface relief.

Previous methods for creating a 3D object by imaging a liquid resin usea photonic principle. Such processes fall under what is known as the“Law of Reciprocity”. This law states that imaging for a long time usinga low intensity light will give the same result as imaging for a shorttime using a high intensity light, as long as the exposure (defined asthe integral of the light intensity over time) stays the same. Adifferent way to state this behaviour is to say that the exposureprocess falls under the law of linear superposition. The law of linearsuperposition states that: f(a+b)=f(a)+f(b). Simply stated, the combinedexposure (a+b) yields the same result as exposure “a” followed byexposure “b”. There are some polymerization processes which deviate fromthe “Law of Reciprocity” such as two-photon absorption processes, inwhich the rate is proportional to the square of the intensity. Suchprocesses still integrate light and suffer from very low sensitivityrequiring high amounts of UV light.

Because of this behaviour, it is not possible to focus an exposure deepinside a liquid resin without also exposing the volume above the desiredexposure point. This is shown in FIG. 1. When beam 3 is focused by lens4 to a point 5 inside liquid resin 1 to polymerize resin 1 at point 5,the area above point 5 will undergo polymerization as well. As point 5moves along a line inside the liquid resin, points in the immediatevicinity of the line along which point 5 moves, the resin are subject tointense exposure for a short time. The volume 6 above the line throughwhich the exposing light passes before reaching point 5 is subjected toa weak exposure for a longer time (due to the large overlap of the beamsforming point 5). Since the product of intensity and exposure is aboutthe same in volume 6 as it is along line 5, volume 6 will polymerize aswell. If the absorbance of material 1 is high, volume 6 will actuallyreceive a higher exposure than the desired area along the line travelledby point 5 as point 5 is scanned to cover a large area. Volumes deeperin the fluid 1 than point 5 will be exposed as well. These volumes willreceive lower exposures since part of the light is absorbed before itreaches those volumes.

For these reasons, prior art systems can only expose the top layer of aliquid polymer and require elaborate means to lower the polymerizedlayer and keep it submerged, in order to build an object layer-by-layer,always exposing only the top layer.

A new class of material known as thermosensitive, or thermal, materialshas become available. Some thermosensitive materials solidify uponheating to a temperature in excess of a threshold temperature.Thermosensitive, or thermal, materials include both polymerizablematerials (“resins”) and coalescent materials. Thermosensitivecoalescent materials typically comprise small particles which coalesceupon the material reaching a threshold temperature. At temperaturesbelow the threshold thermosensitive materials remain fluid. Because ofthis property thermosensitive materials operate completely outside the“Law of Reciprocity” or the principle of linear superposition. Anexample of a thermosensitive process is melting. A block of lead can bemelted by heating it up to 500° C. but cannot be melted by heating it uptwice to 250° C. If kept at 250° C. for even a long time the lead blockwill remain solid. This non-integrating behavior is typical of allthermosensitive materials.

Methods have been known since the 1960's for making printing platesinvolving the use of imaging elements that utilize heat-driven processesrather than photosensitivity. U.S. Pat. No. 3,476,937, Vrancken,discloses a process for making printing plates by imaging particles ofthermoplastic polymer in a hydrophilic binder. The particles coalesceunder the influence of heat, or heat and pressure. This process is usedin heat-based lithographic plates that are developed using variousaqueous media. U.S. Pat. No. 3,793,025, Vrancken, discloses the additionof a pigment or dye to a thermosensitive material in the process ofVrancken '937. The pigment or dye converts visible light to heat. U.S.Pat. No. 4,004,924, Vrancken, further discloses the use of hydrophobicthermoplastic polymer particles in a hydrophilic binder together with alight-to-heat converter. In Vrancken '924, the combination is employedspecifically to generate printing masters by flash exposure. Varioussystems for using thermal coalescing materials to make lithographicprinting plates are known. One example of a thermal coalescing materialused for these purposes is Thermolite™ available from Agfa of Mortsel,Belgium.

Thermal coalescent materials typically comprise a suspension or latex ofparticles which coalesce to form a larger solid mass upon heating.Typically, coalescent materials comprise a suspension of uncoalescedhydrophobic thermoplastic polymer particles mixed with a component whichconverts electromagnetic radiation to heat. Macroscopically, coalescentmaterials appear as a liquid which solidifies locally upon being heatedbeyond a threshold temperature.

Some prior art processes use laser heating for stereolithography bycutting thin sheets or melting a thin layer of powder. However, neitherprocess is suitable for true 3D imaging as the materials used willscatter the light. These processes it can only be used to form objectsin thin layers. Furthermore, in these processes the material starts offas a solid and the heat turns it into a liquid or gas.

SUMMARY OF THE INVENTION

This invention exploits the fact that the exposure of thermosensitivematerials does not obey the law of superposition or the law of linearsuperposition. Three-dimensional objects are created inside a volume ofliquid thermosensitive material by 3D scanning of the volume using afocussed light beam, preferably in the IR part of the spectrum. Thefocussed light beam heats the thermosensitive material to a hightemperature in the immediate vicinity of the focal point. Thethermosensitive material solidifies rapidly at the points the light isfocussed, due to the high temperature, but heats up only slightly in allother areas. As the beam is scanned the areas where temperatures havenot reached threshold of the thermosensitive material cool down. Theexposure is not integrated in these areas. The unexposed parts of thethermosensitive material may be heated repeatedly to temperatures lowerthan the threshold temperature without solidifying.

The efficiency of the process can be further increased by providingmultiple beams, from different directions, which converge on a commonpoint. This also allows nearly constant exposure through the volume ofthe thermosensitive material.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate non-limiting embodiments of the invention:

FIG. 1 is a cross-section of a prior art stereolithography apparatusillustrating the problem of exposing a layer of photosensitive resinlocated below the surface of the resin;

FIG. 2 is a cross-section of a stereolithography apparatus using athermosensitive material;

FIG. 3-a and 3-b show the temperature distribution in a volume of atypical thermosensitive resin during an exposure;

FIG. 3-c shows the polymerization rate a typical coalescentthermosensitive material as a function of temperature; and,

FIG. 4 shows a cross-section of stereolithography apparatus usingmultiple radiation beams being applied to heat up a point inside athermosensitive material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In this disclosure, the term “thermosensitive material” should bebroadly understood as any material capable of being converted from aliquid into a solid by heating to a temperature in excess of a thresholdtemperature. Thermal materials include resins which solidify by way oftemperature-dependent chemical reactions and coalescent materials.

The word “curing” in this disclosure should be understood in a broadsense as any process causing solidification. Curing can be achieved bycoalescing, cross-linking or polymerization but the invention should beinterpreted broadly to include any type of temperature-inducedsolidification of a liquid or gel.

FIG. 2 shows a vessel filled with a thermosensitive material 1. A laserbeam 3, preferably having a wavelength in the IR part of the spectrum,such as 800 nm to 1200 nm, is focussed to a point 5 using lens 4.Non-laser sources could also be used to generate beam 3. Multiple points5 can be generated simultaneously by the use of multi-channel modulatorsor light valves. When light valves are used, they can be one ortwo-dimensional.

Thermosensitive material 1 absorbs the radiation from laser beam 3 (orother light source) and converts it to heat. The thermosensitivematerial 1 preferably includes a radiation-to-heat converter chosen tomatch the wavelength of beam 3. The ability to match the absorbingproperties of the material to the available wavelength makes suchthermosensitive materials more flexible in use than photonic materials.Beam 3 may comprise radiation in almost any part of the spectrum, aslong as an appropriate radiation-to-heat converter is provided in thethermosensitive material. In this respect, a variety of additivessuitable for use as radiation-to-heat converters are available and arewell known to those skilled in the art. The radiation-to-heat convertermay be a suitable dye, pigment or other suitable material capable ofabsorbing electromagnetic radiation and converting it into heat. Theconcentration of the radiation-to-heat converter is selected to permitproper exposure of points 5 at a desired depth with the availableradiation power. Lasers which emit radiation at wavelengths of 800 nm to1200 nm are currently most economical and, as a result, are preferred.

At the point of focus 5 thermosensitive material 1 heats up rapidly andsolidifies. By scanning spot 5 throughout the volume of thermosensitivematerial 1, a 3D object 7 is created. Thermosensitive material 1 doesnot solidify in areas outside the volume of object 7 as in these areasthermosensitive material 1 did not reach the threshold temperature andcooled back down. Even if an area is scanned multiple times there is noaccumulated effect.

FIG. 3-a shows an example of a possible temperature distribution insidea polymerizable thermosensitive material 1 when laser beam 3 is focusedon a spot 5. Because of the high concentration of radiation at spot 5,only thermosensitive material 1 in the immediate vicinity of spot 5 getssufficiently hot to cause thermosensitive material 1 to polymerize. Inorder to produce a very localized effect the angle of the cone ofradiation formed by lens 4 should be large, preferably in the range of30° to 90°. This also increases the optical resolution of the system.FIG. 3-b shows a temperature distribution suited to typical coalescingthermosensitive materials. Coalescing thermosensitive materialsgenerally have lower threshold temperatures than do polymerizingthermosensitive materials.

FIG. 3-c is a graph showing the polymerization rate of a typicalpolymerizing thermosensitive material 1 as a function of temperature. Ingeneral only materials involving a physical change, such as melting,have ideal threshold behaviour in which the material changes its statesuddenly upon being heated past the threshold temperature. Mostpolymerizing thermosensitive materials follow the Arrhenius law and havegraphs of polymerization rate as a function of temperature similar toline 10. The Arrhenius law states that the rate of a chemical reactiongenerally follows the equation:$R = {{Roe}^{- {(\frac{E_{a}}{kt})}}\quad {where}}$ $\begin{matrix}{{R = \quad {rate}};} \\{{{Ro} = \quad {{rate}\quad {constant}}};} \\{{E_{a} = \quad {{activation}\quad {energy}}};} \\{{k = \quad {{{Boltzmann}'}s\quad {constant}}};\quad {and}} \\{t = \quad {{temperature}\quad {in}\quad {degrees}\quad {{Kelvin}.}}}\end{matrix}$

In practice, when E_(a) is sufficiently high, graph 10 is sufficientlyclose to the ideal graph (i.e. a flat graph followed by a steep step atthreshold value 13). For most polymerizing thermosensitive materials, adrop in temperature of 50° C. will slow the reaction by a factor ofabout 30 fold, and a drop of 100° C. will slow the reaction down byabout 1000 fold. For coalescing thermosensitive materials, a small dropin temperature below the material's threshold temperature willdramatically slow down the coalescence rate. In order to make athermosensitive material as sensitive as possible it is desired to makeits threshold temperature 13 as low as possible (but normally above roomtemperature). For Arrhenius type coalescent resins, this conflicts withthe desire to use a coalescent resin which has a long shelf life. Thisproblem can be solved by mixing together two components, such as aliquid comprising a radiation-to-heat absorber on the one hand, and aliquid comprising hydrophobic thermoplastic polymer particles one theother hand, just before use.

Many existing two-component adhesives and casting resins such asepoxies, polyurethane, polyesters and silicone rubber resins can be usedas polymerizable thermosensitive materials. If the amount of catalyst islow the curing time is slow without heating. By adding a suitableradiation-to-heat converter such as an absorber dye or pigment andselectively heating with a laser, curing (polymerization) will onlyoccur at points on which the beam is focused. Another family ofthermosensitive resins is based on Thermal Acid Generators, in which thehigh temperature generates an acid, which serves as a catalyst forpolymerization. These materials are very similar to the existingphotosensitive resins which use a Photo Acid Generator. In the preferredembodiment the laser is a diode laser operating between 800 nm and 1200nm, typically 830 nm. To get the correct absorption the thermosensitivematerial is mixed with a radiation-to-heat converter such as an IRabsorber dye (available from Zeneca Ltd., U.K. and other vendors). Theamount of catalyst and radiation-to-heat converter may be chosen asfollows:

A. After mixing with the catalyst the thermosensitive material shouldstay liquid until the imaging is complete, in order to be able to pouraway the liquid portion of the thermosensitive material. This dictatesusing typically 1%-25% of the amount of catalyst used for normalroom-temperature curing. The exact amount is selected according to thelongest imaging time required. More catalyst allows less time forhandling and imaging but increases the sensitivity of thethermosensitive material.

B. The amount of radiation-to-heat converter is selected according tothe depth of the liquid thermosensitive material. Typical absorbance isbetween A=0.3 to A=3 for the whole thickness (i.e. total transmissionbetween 50% and 1%). Less absorbance creates more uniform polymerizationat lower sensitivity.

C. Many polymerizing thermosensitive materials are exthothermic whenpolymerizing. This is desirable as it increases sensitivity.

D. For best results the thermosensitive material should be prepared justprior to use.

E. It is preferred to build very thick objects up from a few thinnersections, which can be fused together using a resin as an adhesive. Thisallows the use of a higher concentration of radiation-to-heat converter,for greater sensitivity.

In order to achieve more uniform solidification, it is desired to focusthe radiation from different directions. This is shown in FIG. 4. Vessel2 has a window formed by a transparent bottom well 8. Two laser beams, 3and 3′, focussed by lenses 4 and 4′, come to focus at the same spot 5.Spot 5 can be scanned in three dimensions within the volume ofthermosensitive material 1 to create 3D object 7. Lenses 4 and 4′ aremounted on a common frame 9. All other optical components required tofeed radiation beams 3 and 3′ to the scanning lenses are not shown, asthey are well known in the art of laser scanning.

It should be noted that the focal point of beam 3 shifts as it entersthermosensitive material because thermosensitive material 1 has arefractive index higher than air. If beam 3 is passing through athickness d of thermosensitive material, its focal spot 5 will shift byabout d (n−1)/n, where “n” is the refractive index of thermosensitivematerial 1. The focal point of beam 3′ will shift in a similar manner by(h−d)(n−1)/n. Since the sum of the shifts isd(n−1)/n+(h−d)(n−1)/n=h(n−1)/n, which is independent of d, both beamswill always stay in focus as long as the distance between lenses 4 and4′ is compensated by the amount h(n−1)/n. A similar compensation takesplace in the amount of energy reaching spot 5. If the total transmissionof the full thickness “h” is “T”, when spot 5 is at the top or bottom ofthe vessel the amount of radiation is I+I×T=I(1+T), where I is theintensity of beam 3 or 3′. The lowest exposure is when spot 5 is midwaybetween top and bottom. Since the transmission of half of thickness “h”is {square root over ( )}T (where “T” is expressed as a fraction from 0to 1), the exposure in the middle is I{square root over ( )}T+I{squareroot over ( )}T=2I{square root over ( )}T. In order to achieve uniformexposure, 1+T needs to be equal to 2 {square root over ( )}T. This isonly possible for T=1, which means no energy absorbed (and no heating).However, for reasonable values of “T” the exposure stays nearlyconstant. For example, for T=0.5, 1+T=1.5; 2{square root over ()}T=1.42, which is only about 5% less than 1.5. This shows the advantageof the configuration of FIG. 4 over FIG. 2 as radiation intensity willvary by 50% in FIG. 2 for T=0.5.

For better results, radiation intensity can be adjusted automatically.This is done by keeping the power at spot 5 constant by calculating thetotal absorption of each beam as a function of the thickness of thethermosensitive material it is passing through. While the invention isnot limited to any particular application, it is most useful when theobject created is nearly flat, such as relief printing plates. Underthese conditions the invention is more efficient as higher absorption(per unit thickness) can be used. Both letterpress (rigid) andflexographic (flexible) printing plates can be produced. While the mainadvantage of the invention is in the true 3D scanning of a volume ofcoalescent resin, the use of thermoresist has advantages even whenscanning the object one layer at a time, as it allows the use of lowcost and powerful diode lasers instead of expensive UV lasers. In athermal process the wavelength of the laser is of no importance, sinceit is converted to heat. The ability to use low cost lasers plus thefreedom from any concerns of stray radiation or room light is superiorto prior art stereolithography even when exposing only the top layer ofthe thermosensitive material, as done in the prior art.

In some cases, when a polymerizable thermosensitive material is beingused, it can be useful to achieve partial polymerization, turning theliquid into a soft gel, before scanning is done. This allows one, forexample, to form a flexographic plate by coating a dimensionally stablesubstrate, such as a metal or polyester sheet, with such a gel andexposing it by shaping the plate into the form of a cylinder, as acylinder shape is easy to scan by rotating it around its axis. Scanningcan be achieved by a single laser, preferably laser diode, an array oflaser diodes or a light valve used to form many spots from a singlelaser. An additional advantage of using a gel is increased sensitivity,as the resin is already partly polymerized before the exposurecommences. This disadvantage of a gel over a liquid is that the unusedmaterial cannot simply be poured off but has to be removed by washing,scrubbing, etc.

Sometimes a post-curing step is required or desirable to improve theprocess. Post curing steps are usually done after the unsolidifiedportion of the thermosensitive material is discarded (or re-used). Postcuring can include, but is not limited to, the following: baking (i.e.heating of the 3D object in an oven), UV exposure, washing, surface,treating, painting etc. In certain occasions a pre-curing step such asactivation of the liquid thermosensitive material by heating below thethreshold can be used.

In some cases where the object being formed is very thin and does notrequire much vertical resolution, a 3D object can be created by a 2Dscan, using the depth-of-focus of the scanning system to create thethird dimension. Thin flexographic plates can be created this way usingthe invention. By the way of example, a 1 mm thick plate with 0.5 mm ofrelief can be created at a resolution of better than 20 microns, as thedepth of focus of a 830 nm laser beam when focussed to a 15 micron spotexceeds 0.5 mm. For more accurate vertical profiling a 3D scan isrequired; in such a case a smaller spot with lower depth of focus can beused.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. Accordingly, the scope of the invention is to beconstrued in accordance with the substance defined by the followingclaims.

What is claimed is:
 1. A method for forming a three dimensional objectcomprising: (a) providing a volume of a thermosensitive material, thematerial having a rate of solidification which increases withtemperature; (b) raising a temperature of at least one interior spot inthe volume of the thermosensitive material to a temperature sufficientto cause the thermosensitive material to solidify in the vicinity of thespot; and, (c) building a three-dimensional object by repeating the stepof raising a temperature of an interior spot in the volume of thethermosensitive material for different spots.
 2. The method of claim 1,wherein the thermosensitive material solidifies by coalescing.
 3. Themethod of claim 1, wherein the thermosensitive material comprises asuspension of particles which coalesce upon heating to a temperatureexceeding a threshold temperature.
 4. The method of claim 1, wherein thethermosensitive material comprises a suspension of hydrophobicthermoplastic polymer particles.
 5. The method of claim 1, whereinraising a temperature comprises focussing a beam of electromagneticradiation on the spot.
 6. The method of claim 5, wherein thethermosensitive material comprises a suspension of particles whichcoalesce upon heating intermixed with a material capable of convertingthe beam of electromagnetic radiation into heat.
 7. The method of claim1, wherein providing the thermosensitive material comprises mixingparticles which coalesce upon heating with a radiation-to-heat convertercapable of absorbing electromagnetic radiation from the beam andconverting the absorbed radiation into heat.
 8. The method of claim 5,wherein the thermosensitive material comprises hydrophobic thermoplasticpolymer particles and a radiation-to-heat converter capable of absorbingelectromagnetic radiation from the beam and converting the absorbedradiation into heat.
 9. The method of claim 1, wherein building up athree-dimensional object comprises simultaneously focussing two or morebeams of electromagnetic radiation on the spot.
 10. The method of claim9, wherein one of the two or more beams of electromagnetic radiation isincident on the spot through a window located below a surface of thevolume of thermosensitive material.
 11. The method of claim 1 comprisingsimultaneously focussing a plurality of beams of electromagneticradiation on a plurality of spots within the volume of thermosensitivematerial.
 12. The method of claim 1, wherein raising a temperature of atleast one interior spot comprises focussing a beam of radiation on thespot and the thermosensitive material is a material which coalesces uponheating.
 13. The method of claim 12, wherein the thermosensitivematerial is in liquid form.
 14. The method of claim 12, comprisingsimultanteously raising temperatures of a plurality of distinct interiorspots in the volume of the thermosensitive material by the use of alight valve and at least one laser diode.
 15. The method of claim 12,comprising simultanteously raising temperatures of a plurality ofdistinct interior spots in the volume of thermosensitive material bysimultaneously concentrating radiation from one or more diodes in anarray of laser diodes on each of the plurality of spots.
 16. The methodof claim 12, wherein the radiation has a wavelength is in the range of800 nm to 1200 nm.
 17. The method of claim 12, wherein thethermosensitive material comprises a light-absorbing dye.
 18. The methodof claim 12, comprising post-curing the object.
 19. The method of claim12, wherein raising the temperature of the spot comprises focussing ontothe spot two beams of radiation from different directions.
 20. Themethod of claim 19, wherein the two beams are collinear.
 21. The methodof claim 20, wherein providing the volume of resin comprises providing avolume of resin in a container having a transparent wall wherein one ofthe two beams enters the resin through the transparent wall.
 22. Themethod of claim 20, wherein the two beams are focussed by an opticalsystem comprising first and second lenses supported on a common frameand selecting different spots includes moving the frame relative to thevolume of thermosensitive material.
 23. The method of claim 12, whereinrapidly raising the temperature of the at least one interior spot iscarried out while maintaining a temperature of thermosensitive materialoverlying the spot sufficiently low that the thermosensitive materialoverlying the spot does not solidify.
 24. The method of claim 12,wherein focussing the beam of radiation on the spot comprises causingthe beam of radiation to converge on the spot with an angle in excess of30 degrees.
 25. A method for forming a three dimensional objectcomprising: (a) providing a volume of a thermosensitive material; (b)raising a temperature of at least one interior spot in the volume byproviding radiation converging on the spot; (c) allowing the increasedtemperature of the thermosensitive material in the vicinity of the spotto cause the thermosensitive material to solidify in the vicinity of thespot; and (d) building up a three-dimensional object by repeating thestep of raising a temperature of an interior spot in the volume of thethermosensitive resin for different adjacent spots.
 26. A method forforming a three dimensional object comprising: (a) providing a volume ofa thermosensitive material having a threshold temperature wherein thethermosensitive material solidifies rapidly when raised to a temperaturein excess of the threshold temperature; (b) rapidly raising atemperature of an interior spot in the volume of the thermosensitivematerial to a temperature in excess of the threshold temperature byproviding at least one beam of radiation focussed on the spot whereinthe increased temperature of the thermosensitive material in thevicinity of the spot causes the thermosensitive material to solidify bycoalescing in the vicinity of the spot; (c) building up athree-dimensional object by repeating the step of raising a temperatureof an interior spot in the volume of the thermosensitive material fordifferent spots; and, (d) removing unsolidified resin from thethree-dimensional object.
 27. The method of claim 26, wherein rapidlyraising a temperature of an interior spot in the volume ofthermosensitive material to a temperature in excess of the thresholdtemperature comprises maintaining a volume of the thermosensitivematerial surrounding the interior spot at temperatures lower than thethreshold temperature.
 28. The method of claim 26, wherein a rate ofsolidification of the thermosensitive material as a function oftemperature increases sharply the temperature of the thermosensitivematerial is raised past the threshold temperature.